Reviews in Computational Chemistry Volume 18

200 Charge-Transfer Reactions in Condensed Phases
and get narrower than the absorption lines (Figure 17). The opposite trend
holds for negatively solvatochromic dyes with higher major multipoles in their
ground states.
The lower free energy surface has two minima in the normal CT region
(Figure 12). Two absorption transitions exist in this case, even for selfexchange
reactions. The reason is the symmetry breaking induced by a nonzero
adiabatic transition dipole leading to e < 1 (the standard MH picture,
Figure 15, is recovered when m12 ¼ 0). The energy splitting between the two
minima of the lower free energy surface gives rise to two transition frequencies
and
normalized intensity
6
4
2
0
0.7
em
0.8
abs
4 5 6
reduced frequency
Figure 17 The normalized absorption (abs.) and emission (em.) intensities at e ¼ 0:7
(solid lines) and e ¼ 0:8 (long-dashed lines) versus the reduced frequency hn=l I .
The dash–dotted lines indicate the lower boundary for the energy of the
incident light nmin (Eq. [149]).
hn ð1Þ
abs ¼ lv þ l I þ F I s þ e E12 ½151Š
hn ð2Þ
abs ¼ lv þ l I
F I s e E12 ½152Š
The combination of Eq. [134] with Eq. [144] provides an effective formalism
for the band shape analysis of CT spectra when a substantial degree of
electronic delocalization is involved. Equation [134] is exact for a TS donor–
acceptor complex and, therefore, can be used for an arbitrary degree of electronic
delocalization as long as the assumption of decoupling of the vibrational and
solvent modes holds. Figure 18 illustrates the application of the band shape
em
abs